Select all the text layers at once (use command-click if they're spaced out), then go to the Text Inspector and change the font. They *should* all update at once.
*There is an exception: IF you have one of the text layers "pinned", none of the others will update — only the pinned text layer will change its font. [By "pinned", if you look at the top right corner of the Inspector, there is a little "pushpin" icon with an underline. If the pin is touching the line, that layer's parameters are displayed regardless of what other layers may be selected, but you can only change parameter settings for the pinned layer (which can be quite useful sometimes...)]
Another way to "easily" (matter of opinion) change fonts is to change the font for one text object, then click and drag the Font parameter from the Text object and drop it on the other Text layers one by one.